Due to its electrical characteristics being similar to that of a ferrite rod antenna, a speaker coil may be used as an antenna for radio frequency (RF) transmission device, as well as an electromechanical driver for a loudspeaker at audio frequencies. In cost sensitive portable devices such as mobile phones, digital audio devices, headphones and hearing aids, RF functionality can be added along with acoustic functionality without taking up much extra physical space. For example, the addition of RF functionality to the speaker coil within a mobile phone may enable near field communications for financial transactions or for swapping data between users. For hearing aids, the addition of RF functionality may allow the hearing aid to be controlled or programmed remotely, and may enable a binaural pair of hearing aids to exchange data to improve the directionality of the audio signal using audio signal processing algorithms.
In some systems, a combined acoustic and RF transmission system may be implemented by coupling the audio and RF power amplifiers in parallel with the speaker coil. While the RF power amplifier may use AC coupling capacitors to prevent interference to the audio signal, such AC coupling with respect to the audio amplifier may present a capacitive impedance that reflects RF signals generated by the RF power amplifier. In some systems, such reflections may be dealt with by using lossy highpass and lowpass filter networks that may cause loss and attenuation to the audio signal. In some cases, these losses are due to losses in filter inductors used for RF frequencies in the range of 100 kHz up to 30 MHz. Inductors having a very small form factor, such as the ones typically used in portable devices, may be particularly lossy. In other systems, a RF amplifier and acoustic amplifier may be coupled to a speaker coil via a mechanical relay.